1. Field of the Invention
The present invention relates to a heat resisting member which is superior in thermal fatigue resistance, thermal shock resistance and oxidation resistance, and to its production method.
2. Description of the Related Art
In order to enhance the efficiency of high temperature apparatuses represented by a gas turbine for power generation and engines, temperatures at which such components are used are being raised. Accordingly, materials for the component parts of high temperature apparatuses are being required to have enhanced properties, such as a high temperature strength, a high-temperature corrosion resistance and a high-temperature oxidation resistance.
Accordingly, members now being used extensively have a corrosion-resistant and oxidation-resistant metal coating applied onto the surface of a high-strength Ni-based super alloy or Co-based super alloy. And, to enable the use under environments at higher temperatures, a thermal shield coating which enhances the efficiency of cooling a base material has been developed by forming a ceramics layer having a low heat conductivity on the surface of the metal coating. Members applied with such a thermal shield coating are being placed in practical use for, e.g., stationary blades for a gas turbine to be exposed to a low stress loading.
In the above-described heat shield coating, the metal layer to be formed between the base material and the ceramics layer provides a corrosion and oxidation resistance as the metal coating and also relieves a thermal stress produced due to a difference of the thermal expansion coefficients between the base material and the ceramics layer. For such a metal bonded layer, an M--Cr--Al--Y alloy (M is at least one element selected from the group consisting of Fe, Ni, or Co) is often used.
On the other hand, for the ceramics heat shield layer forming the outermost layer, zirconia (stabilized zirconia) which is stabilized by adding rare earth oxide or alkaline earth oxide is used most extensively. Among other ceramics materials, stabilized zirconia has a higher thermal expansion coefficient and also a lower thermal conductivity.
To form the above-described heat shield coating, various types of coating techniques may be adopted. Among such techniques, a plasma spray coating method is used extensively. The plasma spray coating method has advantages that it can use various types of coating materials, its film forming rate is fast, and a thick film can be formed.
However, the conventional ceramics heat shield layer formed by the plasma spray coating method has a disadvantage of being fractured or delaminated easily when it is used for a long period under the environment of causing thermal cycling. It is because cracks are readily formed inside the spray coated layer and such cracks tend to further propagate. Cracks are particularly formed in the neighborhood of the interface with the metal bonded layer where a thermal stress is concentrated. Those cracks formed in the neighborhood of the interface are the main cause of the fractures or delamination of the spray coated layer.
In addition, the ceramics heat shield layer formed by the plasma spray coating method has a disadvantage that the metal bonded layer is easily oxidized when it is used in the oxidizing atmosphere at a high temperature for a long time of period. Such oxidation is caused due to the configuration particular to the spray coated layer. Since a stress is produced as the metal bonded layer is oxidized, the ceramics heat shield layer is separated from the interface with the metal bonded layer.
Furthermore, in the actual environment of using a gas turbine or the like, there is a disadvantage that its members are abraded or damaged due to the collision of large particles or the like. Particularly, the ceramics heat shied layer formed by the plasma spray coating method is heavily damaged by the collision of such large particles, and its surface is readily abraded or damaged. Generally, the spray coated layer has large projections and depressions on its surface, and the adhesion of mutual particles inside the layer is low.
Meanwhile, it is considered to form the ceramics heat shield layer by physical-chemical vapor deposition methods which are represented by an electron beam PVD method (EB-PVD method). But, such film forming methods have disadvantages that a film forming rate is low and a production cost is high as compared with the spray coating method. Besides, when the ceramics heat shield layer formed by a PVD method or a CVD method is used solely, its heat shield effect is low (due to low porosity), and cracks are readily and suddenly formed due to a thermal shock or the like.
As described above, as a method for forming a ceramics layer which serves as a heat shield layer, the plasma spray coating method or the physical-chemical vapor deposition methods which are represented by the EB-PVD method are being used, but such methods have advantages and disadvantages. Therefore, there has not been provided ceramics heat shield layer which satisfies a heat resistant cycling property and a thermal shock resistance of the ceramics heat shield layer, oxidation inhibiting and heat shield effects of a lower layer such as the metal bonded layer, an abrasion resistance, and a resistance to damages due to the collision of flying subjects.